1. Field of the Invention
The present invention relates to a plated steel sheet for a can, and more particularly to plated metals.
2. Description of the Prior Art
A tinplace made by applying tin on a steel sheet electrolytically, tin free steel made by applying chromium or nickel on a steel sheet electrolytically or aluminium sheet and the like have been hitherto used for materials for a food can and a drink can. Among those materials, surface treated steel sheets show enough strength for can use although they are thin. In recent years, there has been often used a convenient can which has so-called easy-open end which can be opened by hand without using any can-opener. The aluminium sheet has characteristic of an easiness of forming easy-open ends.
A body of the can is made of surface treated steel sheet and an end of the can is made of aluminium sheet with the use of the above-mentioned characteristics. However, in case the body and the end of the can are made of different metals respectively, there occurs what is called a bi-metallic corrosion wherein either of those metals dissolves preferentially and corrodes. This bi-metallic corrosion hinders the spread of the convenient can.
The bi-metallic corrosion is a phenomenon that, when metals with different electrode potentials are electrically connected with each other in an electrolyte, electric current flows from a noble metal to the base metal in potential and the base metal ionizes and begins to dissolve. For example, in case a body of a can is made of tinplate and an end of the can of aluminium, the standard potential of aluminium is -1.66 v and that of tin is --0.14 v. In this case, tin is more noble than aluminium. Therefore, an anodic reaction (1) occurs on the surface of the aluminium sheet and aluminum dissolves. At the same time, a cathodic reaction (2) occurs on the surface of the tinplate and hydrogen is produced. EQU Al.fwdarw.Al.sup.3+ +3e (1) EQU 3H.sup.+ +3e.fwdarw.3/2H.sub.2 .uparw. (2)
The above-mentioned reaction does not occur on the whole inner surface of the can because the inner surface of the aluminium end and the tinplate body of the can is usually lacquered. On the side of aluminium, the above-mentioned reaction occurs concentratedly on a defect of a lacquer film. When the reaction proceeds, a hole is made in the aluminium end. On the side of tinplate, produced hydrogen raises the lacquer films causes a blister. If this reaction proceeds, the lacquer films are peeled from the tinplate. This phenomenon occurs very remarkably when chloride ion participates in the reactions.
The reactions are explained above with specific reference to an example of aluminium and tin. However, there is quite a small difference of the case of tin free steel made by applying chromium of nickel on steel electrolytically from the case of tinplate made by applying tin on steel electrolytically. Even if the body of the can is made from aluminium alloy, the same phenomenon can occur in case of a existence of the difference in potential between the end and the body of the can. To overcome these difficulties, the following measures have been taken conventionally.
Firstly, there has been made an attempt to cover carefully the aluminium end with lacquer. This is an attempt to prevent a formation of a call by thickening the lacquer. However, there remain unsolved problems that corrosions are concentrated on the defects of the lacquer films and, therefore, a hole is liable to be made in the aluminium end, and that it is uneconomical to expend much memory for coating the end of the can.
Subsequently, it has been thought to make the potentials of the aluminium end nobler as disclosed in "ANTI-CORROSION" November 1986, p. 4. To accomplish such an object, an attempt has been made to produce an alloy by adding other metal such as copper to aluminium. In this case, the difference in potential between the end and the body of the can needs to be very small and corrosion resistance of the end should be good. A satisfactory alloy, however, has not yet been able to be obtained.